The maximum strength of so-called cold-drawn work-hardened steel wire which is produced by means of cold-drawing down to a final diameter of about 0.2 mm is usually about 320 kgf/mm.sup.2.
In the process of producing such steel wire, the final cold-drawing is performed with the reduction ratio (l n .epsilon.) at nearly 3.2. When, for example, a cold-drawn steel wire of about 0.2 mm diameter is produced from a steel wire rod of 5.5 mm diameter, several repetitions of LP(lead parenting) heat treatment and cold-drawing are required in order to achieve a specific strength.
FIG. 5 shows a typical conventional process flow diagram for production of the cold-drawn steel wire product. According to this process, the 1.2 mm o steel wire of about 125 kgf/mm.sup.2 tensile strength is made from a 5.5 mm o steel wire rod by repetitions of drawing and intermediate LP (dipping the material in a lead bath at about 600.degree. C. after heating it at above 900.RTM. ). The steel wire is further drawn at the drawing ratio mentioned above to produce the final steel wire product which has a 0.2 mm diameter and about 320 kgf/mm.sup.2 tensile strength.
In this process at these conditions, however, further increase of the drawing reduction ratio in order to raise the tensile strength to above 320 kgf/mm.sup.2 is impossible due to loss of ductility of the material.
FIG. 6 shows an example of the relation between the drawing reduction l n (A.sub.o /A.sub.n), and the consequent tensile strength and RA (reduction in area), where A.sub.o stands for the cross sectional area of the steel wire before drawing, A.sub.n for that after n times (n passes) drawing, and .epsilon. is A.sub.o /A.sub.n.
As is shown in FIG. 6, the strength of the drawn wire product gradually increases as the process of drawing proceeds.
When a conventional steel wire of eutectoid composition with 1-2 mm diameter is cold-drawn and combined with LP treatment, the strength arrives at the maximum value of about 320 kgf/mm.sup.2 at l n .epsilon.=3.2, as mentioned above.
We inventors have disclosed in Japanese Patent Publication No.3-240919 a method of producing a steel wire for making the cold-drawn wire product, wherein the steel wire rod with 0.7-0.9% carbon is heated to austenite temperature above Ac .sub.3 point, then cooled to a temperature range below Ae.sub.1 point and above 500.degree. C. at the cooling rate that would not come across the pearlite transformation starting temperature, to produce a steel wire having subcooled austenite. Thereafter, the steel wire is transformed after cold working with a cross-sectional area reduction of over 20%.
According to the method disclosed in the above mentioned Japanese Patent Publication, crystallographic grains (pearlite blocks) are refined to about 5 .mu.m by thermomechanical treatment, and the separation distance between pearlite lamellars is controlled to a coarseness of about 0.15 .mu.m. Therefore, the obtained steel wire for cold drawing has a tensile strength grade of 115 kgf/mm.sup.2. The cold-drawn steel wire product made from the steel wire can have a tensile strength of about 410 kgf/mm.sup.2 by finally drawing at a reduction ratio close to l n .epsilon.=4.9.
In the process of Japanese Patent Publication No.3-220919, however, due to delayed recovery and obstructed recrystallization of austenitic structure, excessive amounts of residual deformed structure causes generation of free ferrite grains during pearlite dissociation process. The ferritic structure is a factor that inhibits attaining high strength in the final drawing process, due to a loss of ductility and insufficient work hardening.
For this reason, the maximum tensile strength of the cold-drawn steel wire product is limited to 410 kgf/mm.sup.2 grade, even if the 115 kgf/mm.sup.2 level steel wire is cold-drawn at a working ratio close to l n .epsilon.=2.9.
Furthermore, such a high working ratio tends to generate internal defects, subsequently lower the ductility of the wire product, and deteriorate its fatigue strength.